When a digital broadcast recorded or a content stored on a BD or a DVD is played back, subtitles, running commentaries, or the like sometimes appears on the screen. In the following description, subtitles will be used as a typical example of those various types of text information. In that case, the video device has generated an output image so that graphics representing the subtitles are superimposed on a video signal.
The video signal representing the digital broadcast received or the content stored on a BD or a DVD may have been subjected to a 4:2:0 sampling and then compressed and encoded compliant with one of the MPEG standards.
As used herein, the “4:2:0 sampling” refers to a technique for generating color difference signals (including Cb and Cr signals) at a rate that is a half as high as a luminance signal (Y) both horizontally (i.e., along the scan lines) and vertically with respect to the video on the screen. For example, take a matrix consisting of four pixels (i.e., two vertical pixels by two horizontal pixels) as an example. In that case, the luminance signal (Y) is generated for each of the four pixels, while the color difference signal (which may be either the Cb signal or the Cr signal) is generated for only a representative one of the four pixels. A video signal that has been subjected to such 4:2:0 sampling will be also referred to herein as a “4:2:0 format video signal”.
The player usually demodulates the compressed video signal, generates graphics data representing the subtitles as an OSD (on screen display) plane video signal (which will be referred to herein as an “OSD video signal”), and then synthesizes together the video signal and the OSD video signal in accordance with their degree of transparency information to carry out rendering. This synthesis process is performed after the video color difference signals are subjected to vertical filtering and transformed into color difference signals (which are signals in either 4:2:2 format or 4:4:4 format) that have the same phase (which will be referred to herein as “vertical phase” or “sampling phase”) as the video luminance signal (see patent document 1, for example).
To get the given synthetic video signal matched to the resolution of a display device connected, a player performs scaling processing on the synthetic video signal and then outputs the processed signal to the display device. As this scaling processing, IP (interlace/progressive) conversion is sometimes carried out. When the IP conversion is performed, the color difference signals of the synthetic video signal are also subjected to the vertical filtering.
Portions (a) through (d) of FIG. 7 illustrate an example of conventional video processing to be carried out on a synthesized color difference signal. In this example, interlaced video with 1,080 horizontal scan lines (which will be referred to herein as “1080i video”) is transformed into, and output as, progressive video with 720 horizontal scan lines (which will be referred to herein as “720p video”). The 1080i video is supposed to be compliant with the 4:2:0 format. In the following description, however, the luminance signal will be neglected. Also, in the following description, to clearly indicate that the video signal is processed on a pixel-by-pixel basis, each pixel value of the color difference signal will be referred to herein as “color difference data”.
Specifically, portion (a) of FIG. 7 illustrates top and bottom fields of the 1080i video. In this case, the top field color difference data, which are indicated by the solid circles ●, may form a Cb signal, while the bottom field color difference data, which are indicated by the open circles ◯, may form a Cr signal, for example.
Portion (b) of FIG. 7 illustrates 4:2:2 format video signals obtained by subjecting the top-field and bottom-field color difference signals shown in portion (a) of FIG. 7 to vertical filtering. Each of the arrows that connects one of the color difference data (●) shown in portion (a) of FIG. 7 to multiple ones of the color difference data (●) shown in portion (b) of FIG. 7 indicates on which color difference data shown in portion (a) of FIG. 7 the color difference data of each field shown in portion (b) of FIG. 7 has been generated. It can be seen that the amount of information of the color difference signal in each field has been doubled as a result of the vertical filtering.
Such vertical filtering is carried out because if the number of pixels of a color difference signal is equalized with that of pixels of a luminance signal, the IP conversion processing can be carried out more easily after that.
Portion (c) of FIG. 7 illustrates a color difference signal that has been subjected to the IP conversion processing. As a result of the IP conversion processing, the top-field color difference data and the bottom-field color difference data have been integrated together to generate a single frame of color difference data. That is to say, by performing the IP conversion processing, a color difference signal (1080p) representing a progressive frame has been generated. After that, scaling processing (which is also a kind of vertical filtering) is carried out to reduce the number of scan lines to 720.
Portion (d) of FIG. 7 illustrates a frame color difference signal (720p) that has been subjected to the scaling processing. Each of the arrows that connects one of the color difference data shown in portion (c) of FIG. 7 to its associated ones of the color difference data shown in portion (d) of FIG. 7 indicates on which color difference data shown in portion (c) of FIG. 7 each scan line color difference data of the frame shown in portion (d) of FIG. 7 has been generated.
[Prior Art Document]
[Patent Literature]
[Patent Document 1] PCT International Application Japanese National-Phase Laid-Open Patent Publication No. 10-501942